CQI or Channel Quality Indicator is used to adapt the rate for a radio link in e.g. High Speed Downlink Packet Access (HSDPA). The user equipment (UE) measures the quality of the radio channel, i.e. a function of SINR and reports that value or a function thereof to the network. The network scheduler may then use this value to adapt the transmission parameters, e.g. modulation order and coding rate so that the UE can decode the data with a given error probability.
When introducing MIMO (multiple input multiple output) transmission/reception capabilities, the link adaptation, and hence CQI reporting may be handled separately between the multiple streams of data transmitted during one transmission time interval (TTI). In this case the UE will measure the CQI per data stream and report this back to the network. The network may then use this information to perform link adaptation of the streams. It has been noted that this kind of separate adaptation is far from optimal. It is also well known that when the channel conditions deteriorate, fewer parallel data streams may be transmitted. This is sometimes referred to as rank adaptation of the transmitted signal.
For example, if two Tx and Rx antennas are used in the system, a maximum of two separate streams may be transmitted. But for situations with e.g. low signal-to-noise ratio (SNR), the channel will only support one transmitted stream. The final decision of how many streams to transmit will be taken by the system scheduler, and this will depend, not only on the instantaneous channel conditions, but also on available system resources, amount of data in the scheduling buffer etc.
Rank selection will heavily influence the CQI estimation. If two streams are expected in the terminal, it will calculate CQI (SINR) per stream taking into account that there is data on the other stream that will interfere. This means that the channel quality that the user has estimated takes into account the interference caused by multi stream transmission. If the scheduler now needs to schedule only one data stream for some reason (lack of resources etc.), it can not perform proper link adaptation because the CQI for single stream transmission is not known, and can not easily be derived by the network.
One solution to this problem is that the UE will not only report CQI for multi stream transmission, but also for single stream transmission. That is, the UE reports the quality as if a single stream is scheduled and the quality as if multiple streams are scheduled. This is, at least in principle, possible at least if the maximum number of streams are low (e.g. 2). But if a larger amount of streams can be allocated, say for 4, it has to report the quality (per stream) for 4 different cases.
This will increase the uplink (UL) signaling amount and is in practice not possible. There are other solutions as well, the different CQI may be fed back in a time division multiplexing (TDM) fashion, meaning that they are time multiplexed on the UL signaling channel. This has the drawback that the adaptation rate will be slower. The system can not follow the variations in the channel to the same extent.
Similar problems as described above will arise when the system uses Spatial Division Multiple Access (SDMA) to transmit a lower number of streams to several users. Normally, MIMO is envisioned as transmitting several data streams to one user, and thus increase the peak rate. It is also possible to divide the available data streams among several users. In this case, increased system capacity is targeted rather than increased peak rate. When to apply which mode, is up to the scheduler, and in principle such a decision can be taken on TTI by TTI basis.
The problem for the scheduler in this case is that it will schedule, say one stream to each user, but needs a CQI that reflects the interference that will arise due to the multi-user transmission.